Production of ethanol by microorganisms provides an alternative energy source to fossil fuels and is therefore an important area of current research. It is desirable that microorganisms producing ethanol, as well as other useful products, be capable of using xylose as a carbon source since xylose is the major pentose in hydrolyzed lignocellulosic biomass. Biomass can provide an abundantly available, low cost carbon substrate. Zymomonas mobilis and other bacterial ethanologens which do not naturally utilize xylose have been genetically engineered for xylose utilization by introduction of genes encoding 1) xylose isomerase, which catalyses the conversion of xylose to xylulose; 2) xylulokinase, which phosphorylates xylulose to form xylulose 5-phosphate; 3) transketolase; and 4) transaldolase (U.S. Pat. Nos. 5,514,583, 5,712,133, 6,566,107, WO 95/28476, Feldmann et al. (1992) Appl. Microbiol. Biotechnol. 38: 354-361, Zhang et al. (1995) Science 267:240-243; Yanase et al. (2007) Appl. Environ. Mirobiol. 73:2592-2599). Typically the coding regions used were from E. coli genes.
Even with expression of this xylose utilization pathway, typically the engineered strains do not grow and produce ethanol as well on xylose as on glucose. Strains engineered for xylose utilization have been adapted by serial passage on xylose medium, resulting in strains with improved xylose utilization as described in U.S. Pat. No. 7,223,575 and U.S. Pat. No. 7,741,119. The latter also discloses inactivation of the GFOR locus encoding glucose-fructose oxidoreductase to improve xylose utilization. Disclosed in U.S. Pat. No. 7,998,722 is engineering for improved xylose utilization by expression of E. coli xylose isomerase from a mutated, highly active Zymomonas mobilis. glyceraldehyde-3-phosphate dehydrogenase gene promoter (Pgap).
There remains a need for engineered strains of Zymomonas and other bacterial ethanolagens with improved xylose utilization and ethanol production in xylose-containing medium, and processes for using these strains to produce ethanol.